buildpkg.packaging.solaris: add more arch and os helper vars

These used to be defined in gcc/build.sh.gcc.common.
buildpkg.packaging.solaris: distinguish between arch and build_arch

This allows building for targets like sparc64-sun-solaris2.9 by
defining build_arch as sparc64.
buildpkg.functions: change how pkgdirdesig and topinstalldir is defined

Instead of defining pkgdirdesig using topinstalldir do it the other way
around.
build.sh.functions: use build_arch in logfile name